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COMETS by Alan MacRobert
A Revival of Comet Holmes
Update Dec. 11: The bright, glowing blob that many people saw last night and some mistook for a comet was a fuel dump from an US Atlas Centaur rocket that had just launched a spy satellite. Details.
Did I say a few days ago that Comet Holmes had become invisible to the naked eye? That was then; now it’s back.
The difference isn’t in the comet. It’s that the Moon has just left the early-evening sky, allowing a dark view for the first time in nearly two weeks.
Around 7 last night (November 27th), I stepped out my front door in Boston’s moderately light-polluted outer suburbs, looked up, and there it was again a sizeable puffglow just above Alpha Persei (Mirfak). Not only is the moonlight gone, but now the comet is separated enough from Alpha Per that the star’s 2nd-magnitude glare doesn’t get in the way.
Binoculars actually gave a prettier view than my 12.5-inch reflector framing the big, parabola-shaped comet head (a good 0.7° wide, I estimated) next to the gorgeous starry field of the Alpha Persei Association. Inside the head was the familiar largish glow behind the (invisible) nucleus and the broad “spine” that’s now extending behind the glow. The 12.5-inch at 75× showed these features hardly any better, so big are they now, and the whole thing overfilled the telescope’s view.
I’ve long thought that moonlight and light pollution differ in how badly the same amount of them hinder astronomical views, with light pollution being the worst. But I may have to revise that notion considering the experience of my S&T colleague Tony Flanders observing the comet last evening from a badly light-polluted inner suburb of Boston. Here’s what he says:
“I got my best naked-eye view of Comet Holmes to date. When the comet first exploded in October, it was extremely bright but nearly stellar. Later on, it got tangled with Alpha Persei and the surrounding group of bright stars. And when the full Moon was nearby, I couldn’t see the comet at all without binoculars.
“But now it’s much easier to spot naked-eye than any deep-sky objects except the Pleiades. It’s big enough so that nobody could mistake it for a star, and it appears significantly brighter than the Andromeda Galaxy or the Double Cluster.
So, is moonlight gentler or tougher on deep-sky observing than the same amount of light pollution? What do you think? Add to the comments below.
P. S.: I can think of two possible reasons for a difference between their effects:
Color. A light-polluted sky has the peach-orange tint of high-pressure-sodium lamps (though it’s been turning whiter in the last decade with the spread of metal-halide lamps). A moonlit sky, on the other hand, is blue exactly as blue as a daytime sunlit sky, as long-exposure photos show (try it). We don’t see this because our color vision doesn’t work well in dim light, but it might affect the visibility of faint things even so.
Altitude. Moonlight comes from the top down, illuminating the upper atmosphere as much as the lower atmosphere. Light pollution comes from the bottom up. Much of it spreads out from limited areas (cities), and the worst of it consists of near-horizontal rays; these characteristics mean that it illuminates the lower atmosphere more than the upper.
Posted by Alan MacRobert, November 28, 2007